In conventional wind turbines, to change an orientation of a wind turbine in accordance with change in a wind direction, an active control apparatus which is combined with a wind direction sensor is used. For example, there is employed a configuration that a wind turbine is turned by a power apparatus in accordance with a result of measurement of the wind direction sensor, and the wind turbine is held at a position suitable for the wind direction. To simplify a system of the entire wind turbine, the active control apparatus is omitted in some cases. When the active control apparatus is to be omitted, a rotation shaft of a rotor of the wind turbine is supported on a horizontal plane such that the rotation shaft can freely turn, and the orientation of the wind turbine is changed by a weathercock effect to follow the change in the wind direction.
In the wind turbine, the rotor receives wind and rotates, thereby generating electric power. If moment in a vertical direction is applied to a rotation axis of rotation of the rotor, gyro moment is generated in a direction cross at right angles to both the direction of the moment and the rotation axis of the rotor by a so-called gyro effect. For example, in a floating offshore wind turbine provided on a floating body which is floating in water, moment in the vertical direction is generated due to influence of waves. Hence, gyro moment is generated in a horizontal direction cross at right angles to the rotation axis of rotation of the rotor by the gyro effect.
In a floating offshore wind turbine including the active control apparatus, a nacelle is held by a floating body at a position matching with a wind direction. Therefore, rotation motion of the floating body rotating around a central axis of the floating body is generated by gyro moment caused by the gyro effect generated in the nacelle in which the rotation shaft of the rotor of the wind turbine is accommodated. Here, since motion of waves is repetitive motion, the floating body moves repetitively (yawing) together with the nacelle.
In a floating offshore wind turbine in which the active control apparatus is omitted, a rotation shaft of a rotor of a wind turbine is supported such that the rotation shaft can freely turn with respect to a floating body. Hence, yawing of a nacelle is generated by gyro moment caused by a gyro effect generated in the nacelle.
The present inventor found that moment caused by this gyro effect was a cause of an adverse influence exerted on power generating efficiency of the floating offshore wind turbine and endurance of devices thereof.
To prevent vibration generated in a wind turbine apparatus, it is proposed to employ various configurations (patent documents 1 to 3).
Patent document 1 describes a wind turbine apparatus. The wind turbine apparatus includes, as an active control apparatus of a nacelle, a whirling driving source which whirls a platform supported on an upper end of a tower and fixing means in a whirling direction. In this wind turbine apparatus, to suppress vibration generated in the tower or the like by resonance of a blades and resonance wind speed, patent document 1 describes a configuration that a vibration suppressing apparatus is provided.
In a wind turbine, to attenuate vibration action in an edge direction of blades of a rotor, patent document 2 describes a configuration that oscillation action attenuating means is disposed in a nacelle.
In a wind turbine device, to prevent vibration from being transmitted to a nacelle frame through a speed increasing gear box and to prevent vibration from being transmitted from the nacelle frame to the speed increasing gear box, patent document 3 describes a configuration that a vibration isolation damper is provided between the speed increasing gear box and the nacelle frame.